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Waste Management - WBM and drilling fluid performance

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Water Based Mud (WBM) systems have been the traditional drilling fluids since the early days of the oil industry. Deeper, hotter wells, more deviated and with increasing well path tortuosity have brought the WBM systems to their limits of economic applicability.

1 Introduction

WBM systems are ranging from the classic Water-Bentonite suspension via the highly dispersive Lignosulphonate drilling fluids to the KCl-Polyglycol drilling fluid probably the most inhibitive WBM. Silicate Mud, may be the ultimate WBM system with the same inhibitive properties as OBM. Also, it is assumed that no performance difference exists between OBM and synthetic mud, hence OBM is used as reference to OBM or synthetic mud.

In this article we present evidencethat in the North Sea the usage of conventional WBM systems in many hole sections is possible without the fear that drilling performance is adversely effected.

Performance Indicators

Quantifying the success of a drilling fluid is difficult as many factors play a role in successfully drilling a well section. The time-depth graph is widely used as a Performance Indicator (PI). The shorter the time to complete a well section the better, provided the agreed drilling programme is executed effectively. A second PI may be the Stuck Pipe Frequency and the associated time to free the pipe: Recovery Time (RCT). Instances of stuck pipe will then require the section to be re-drilled, effectively doubling the Open Hole Time.

2 Results

Analysis of the historical performance data for WBM and OBM indicates that in 17.5" sections the following variables effect the drilling performance derived from drilling fluids: shale reactivity, section length, mud type.

For the analysis, the 17.5" footage planned  was used as the base case. Sections drilled with WBM were then compared with the same sections drilled with OBM. The results indicate that a section less than 4400ft drilled in unreactive shale with WBM will have a 90% expectation of having an Open Hole Time (OHT) exceeding the average Open Hole Time (OHT) for drilling the section in a reactive shale with OBM by 17%.

From the historical results it becomes apparent that 17.5" sections in unreactive shales can be drilled with WBM without incurring a performance penalty. The performance seems to be slightly dependant of section length (4,400ft is proposed as a critical length) although this is considered as less significant. Hole angle does not seem to play a major role mainly although that could not be assessed properly as the data set is synthetic mudall.

From the analysis it could also be concluded that the value for the critical Open Hole Time (OHT) currently in use for 17.5" sections (7 days) is far too conservative and longer Open Hole Time (OHT) can be allowed, in particular for sections that will be drilled through medium or non-reactive shales.

3 Silicate Drilling Fluid Performance

The field results shown that for medium reactive shales the Open Hole Time (OHT) performance of silicate mud is in the range of conventional WBM performance. It is important to appreciate, however, that the expected benefit of using silicate mud, rather than conventional WBM, lies in improved hole stability and hence reduced Stuck Pipe Frequency (SPF) and Recovery Time.

Logging results from these trial sections, however, have shown much improved shale stabilisation, with good in gauge hole. The expectation is, therefore, that the silicate WBM may show a significant performance improvement for drilling the highly reactive shales currently incompatible with WBM.

4 Conclusion

From statistical analysis of mud performance data from 17.5" sections it has been established that:

  • a split should be made between reactive and non-reactive shales. Reactive shale areas have been identified as Dunlin, Osprey, Fulmar and Auk (highest Stuck Pipe Frequency for WBM). Medium or non-reactive shales may be drilled with conventional WBM systems or with the Silicate mud.
  • reactive shales need to be drilled with OBM, unless field trials with the silicate mud system show improved performance when compared to conventional WBM. Continued silicate mud development and support for field trials is strongly recommended.
  • hole angle is not seen to play a major role: this needs to be further assessed.
  • indications are that in 17.5" sections a critical section length (4400 ft) plays a role. This needs to be further investigated.